If you save different exposures from one Raw file then you don't actually get different dynamic ranges as you just use the same information for all exposures. If you take different exposures with your camera then each shot will have unique information and you'll get a higher dynamic range. With exposures from the same shot you get 16 bit (if you use tiff) and with different exposures you get 32 bit, when doing HDR. So what you've done here isn't really HDR, it's just tone mapping. But it's pretty much the same thing.